A variety of disease states result in altered heart function and a correlated alteration in the cardiac myosin ATPase activity. In particular, treatment of animals with the thyroid hormone thyroxine leads to induction of ventricular myosin with elevated CA++ and Mg+2 ATPase activities and a corresponding increase in the rate of tension development with the ventricular wall-stress velocity having a rightward displacement as a function of afterload. To date we have studied the transient kinetics of normal and thyrotoxic cardiac myosin and established that one rate constant (k4, the rate limiting step) and the aggregation properties of thyrotoxic myosin are altered relative to normal myosin. The studies proposed here are designed to determine if any steps in reaction mechanism other than those investgigated to date are altered by thyrotoxicosis and to conduct studies to deduce the molecular basis of the differences between normal and thyrotoxic cardiac myosin as well as the difference between skeletal and cardiac myosin. In addition a series of experiments are proposed to explore the aggregation properties of normal and thyrotoxic cardiac myosin and to investigate the "isozyme" distribution in these preparation. The studies proposed use primarily transient kinetic techniques (stopped-flow, pressure-jump and temperature-jump). In order to derive information relative to the molecular basis of the difference between normal and thyrotoxic cardiac myosin we will focus on the effect of pH, ionic strength as well as the properties of different nucleotides and chemical modifications (SH1 thiol). Thus we will derive an understanding of the chemical and structural properties important in the mechanism of ATP hydrolysis by myosin. Finally through a comparative study we will be able to describe those chemical and structural properties altered by thyrotoxicosis. The studies described will provide a quantitative characterization of cardiac myosin and lay a foundation for the ultimate understanding of the molecular basis of cardiac disease as reflected in myosin.